Field
The present disclosure relates generally to surge arresters, isolators, bushings and other high voltage distribution system components for use in high voltage power distribution systems, and to systems for monitoring the temperature of surge arresters, isolators, and bushings without affecting the performance of the surge arresters, isolators, and bushings.
Description of the Related Art
High voltage power distribution systems use substations as part of the overall electrical generation, transmission, and distribution system. A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages. Between a power generating station and consumers, electric power may flow through several substations at different voltage levels. On the consumer side of the overall electrical generation, transmission, and distribution system, transformers are used to step down the voltage on the power lines for customer usage.
The purpose of a surge arrester is to divert damaging over-voltage transients (i.e., surges) caused by lightning or switching events, safely to ground, thereby protecting the transformer or other system equipment from damage. Current surge arresters typically include a housing, a connection for a high voltage line on one end, and a connection on the other end for grounding the arrester to the earth ground, and a series of metal-oxide non-linear resistors within the housing between the two connections. Metal-oxide non-linear resistors are sometimes called blocks, zinc oxide discs or blocks, metal oxide blocks, or metal-oxide surge arresters. The one or more blocks of non-linear resistors form one or more varistors. Typically, the ground connection side of the surge arrester is connected through a ground-lead disconnector that is integral to or separately attached to a pedestal of isolator bracket.
Metal oxide surge arresters typically have very high reliability and can continue to function as intended for long periods of time if they have been well designed, properly manufactured, and operated within their specified range of applied voltage, temporary overvoltage, surge magnitude and surge energy, and have not been physically damaged by some external force. However, metal-oxide surge arresters do occasionally fail while in service thereby negatively impacting the power distribution system. A surge arrester failure occurs when the arrester is no longer able to support the normal high tension line voltage, and such failure often results in a short circuit on the power system. Most, if not all, surge arrester failure modes have the same end-of life condition—a short circuit. There are various ways in which an arrester can deteriorate to the point of ultimate failure on the power system. Examples of ways in which surge arresters can fail include; moisture ingress to the interior of the arrester, an overvoltage that remains beyond a rated time the metal oxide block temperature is designed for, and block aging.
Surge arrester failures typically result in an operation of a circuit interrupting device (e.g., a circuit breaker, recloser, fuse, etc.), causing disruption of electrical service to consumers. If the surge arrester is equipped with a disconnector, the disconnector should operate when the fault current flows following a surge arrester short circuit, isolating the surge arrester either from the ground or from the line, depending on where the disconnector is installed. Upon re-energization, the failed surge arrester is no longer in the circuit electrically and service can be continued, but with a reduced level of surge protection. In the case of a substation arrester, a failure typically causes a system lockout until either that portion of the substation can be bypassed, or until the surge arrester is physically removed from service. While it is desirable to avoid substation and distribution arrester failures, the consequence of a substation arrester failure is typically much more severe than that of a distribution arrester. Often, a considerably greater disruption of service would accompany a substation arrester failure, and the cost of replacing a substation arrester is typically orders of magnitude greater than that of replacing a distribution arrester.
As a precautionary measure, utilities attempt to monitor surge arresters in the hope that they could detect deterioration in a surge arrester, and remove the surge arrester from service and replace it with a new surge arrester on a planned basis before a costly failure of the surge arrester were to occur.